1. Field of the Invention
The present invention relates to a vehicle door frame structure, and in particular relates to a joining structure between an upright pillar sash extending in the vertical direction of the door and an upper sash, which forms the upper edge of the door.
2. Description of Related Art
A vehicle door frame in which mutually opposed longitudinal-end surfaces of an upright pillar sash, extending in the vertical direction of a door, and an upper sash, which forms the upper edge of the door, are butted and joined together by welding in the door corner portion is known in the art. The butting and joining of the mutually opposed longitudinal-end surfaces requires a high-precision manufacturing process of the parts so as to have a low tolerance in each profile (shape) of the mutually opposed longitudinal-end surfaces, and hence, it is difficult to achieve a favorable joining strength therebetween.
Japanese Unexamined Patent Publication No. 2007-137200 discloses a door corner structure in which the end-surface of an upper sash butts against an upright pillar sash while part of the upper sash is inserted inside the upright pillar sash to overlap therewith. Due to the insertion portion of upper sash that is inserted into the upright pillar sash, ease of positioning and improvements in the joining strength, to a certain extent, can be anticipated with this joining structure; however, an improved door frame structure is desired with respect to the strength and precision of the door corner portion.